1. Field of the Invention
The present invention relates to an amplifier and, more particularly, to a high-frequency power amplifier having a heterojunction bipolar transistor.
2. Description of the Related Art
A heterojunction bipolar transistor (hereinafter referred to as "HBT"), which is characterized in that its emitter and base are mutually connected, thus forming a heterojunction, is superior to a homojunction bipolar transistor in switching speed, high-frequency characteristic, and the like. This is because: (1) since the emitter and the base form a heterojunction, the emitter injection efficiency is high; (2) the impurity concentration of the base can be increased, thus reducing the base resistance; and (3) the impurity concentration of the emitter can be reduced, thereby decreasing the emitter junction capacitance. Hence, if an HBT is incorporated into a power amplifier, the amplifier can provide efficient power amplification in a high-frequency region.
Most HBTs have a heterojunction structure made of a GaAlAs/GaAs semiconductor material. Such an HBT has a relatively high turn-on voltage, however, such an HBT has not provided for great power gain in a power amplifier. This problem renders it difficult to realize a high-efficiency power amplifier circuit which includes an HBT.